This invention relates to the measurement of at least one of brightness, flow velocity or temperature of radiant media In one particular application the invention relates to measurement of the brightness, flow velocity and temperature of optically thin radiant media such as high and low temperature plasma discharges, flames, aurora, solar corona etc. In other applications the invention can, for example, be used to measure temperature of black body radiators such as furnaces, smelters and the like.
Standard spectroscopic methods for measuring flow speeds and temperatures of excited atoms or ions utilize grating spectrometers for dispersion of the emission spectrum together with intensified CCD detector arrays to resolve the emission line profile. Numerical fitting of the data allows estimates of the Doppler shift of the emission line due to the bulk flow velocity and spectral broadening due to the temperature of the source.
An instrument measurement of solar velocity and magnetic fields has been proposed by Stenflo (Applied Optic, 23:1267-1278, 1984). The Stenflo spectrometer obtains a phase delay xcfx860 using a beamsplitting cube to separate the component polarizations and different thicknesses of glass to relatively delay these waves. Wavelength specific quarter wave plates are used to return the separated beams in such a way that they traverse a path orthogonal to the incident path. Modulation is achieved using a fixed-frequency photo-elastic modulator (PEM) which is a stand alone commercially available component. A half wave plate is used to tune the phase offset xcfx860.
The Stenflo instrument contains at least ten independent optical components five of which are wavelength specific. This presents serious practical limitations to the use of the instrument. Additionally the Stenflo instrument has not been proposed for the measurement of source temperature. Although the Stenflo instrument could be used for temperature measurement the light wavefront separation at the beamsplitter can introduce relative phase disturbances which, upon recombination of the beams, will reduce the ultimate temperature resolution.
It is an object of this invention to provide an improved method and apparatus for measurement of the brightness, flow velocity and temperature of radiant media.
The present invention proceeds from a recognition that the low order spectral moments of the line-of-sight integrated emission from excited atoms or ions are related by the Radon transform to moments of the species velocity distribution. The light emission integrated along some line-of-sight L as a function of emission frequency xcexd is represented by                               g          ⁡                      (            v            )                          =                              ∫            L                    ⁢                                    I              ⁡                              (                                  r                  ,                                      l                    ^                                    ,                  v                                )                                      ⁢                          ⅆ              l                                                          (        1        )            
where {circumflex over (l)} is the unit vector in the direction of view. The zeroth spectral moment is simply g(xcexd) integrated over frequency xcexd and is proportional to the number of emitting particles along the line-of-sight. The first spectral moment (spectral shift) measures the intensity weighted component of the bulk flow velocity in the direction of and integrated along the viewing line. The second moment (spectral width) is proportional to the intensity weighted temperature of the emitting species. The line integral measurements can be inverted tomographically to give the spatial distribution of the emission, the flow vorticity and the temperature of the source.
For the special case of a gas or plasma in thermal equilibrium, the velocity distribution of the atoms or ions is described wholly in terms of its three lowest order moments.
Accordingly, in one aspect this invention provides an apparatus for measurement of brightness, flow velocity and temperature of radiant media including means to direct a substantially collimated beam of light having selected frequency from said media to a linear polariser;
an electro-optically active birefringent crystal to separate a linearly polarised output of said polariser into two characteristic waves and to introduce a first fixed phase delay between the characteristic waves;
means to selectively electro-optically modulate said birefringent crystal to introduce a second variable phase delay between said characteristic waves; and
means to combine said characteristic waves to interfere.
Preferably, the birefringent crystal is arranged with its propagation axes at 45xc2x0 to the polarisation of the input so that substantially equal characteristic waves are transmitted.
The characteristic waves are preferably combined using a further linear polariser.
A further tuning crystal can, when required, be introduced between the birefringent crystal and the means to combine in order to adjust the amount of first fixed delay.
In a development of the invention two such tuning crystals can be introduced in series and mutually oriented at 45xc2x0 or rotated at a selected rate perpendicular to its optical axis so as to provide a scanning of the delay range. In this application the apparatus can be used as a limited fourier transform spectrometer.
Preferably the birefringent crystal is a Lithium Niobate crystal. Other suitable crystals include BBO and KTP.
The apparatus of this invention preferably further includes means to detect the combined characteristic waves and means to sample the wave at an interval of one quarter of the modulation period of the birefringent crystal to produce a digitised signal from which the emission moments of the radiant media can be recovered algebraically.
In a second aspect this invention provides a method for measurement of brightness, flow velocity and temperature of radiant media including the steps of linearly polarising a substantially collimated beam of light having selected frequency from said radiant media;
separating said polarised beam into two characteristic waves and introducing a first fixed phase delay between said characteristic waves;
introducing a second modulated variable phase delay between said characteristic waves of frequency xcexa9 and amplitude xcfx80/2; and
combining said characteristic waves and sampling the combined wave at an interval of one quarter of the modulation period to produce a digitised signal from which the emission moments of the radiant media can be recovered.
Preferably, a birefringent element is used to introduce the fixed delay. More preferably, an electro-optically active birefringent crystal is used to introduce both the fixed delay and the variable delay.
Preferably, the emission moments are recovered using isolating time domain filters.
As used in this specification the term xe2x80x9clightxe2x80x9d is intended to include ultraviolet and infrared radiation as well as light in the strictly visible spectrum.
In accordance with the method of this invention a novel processing algorithm is employed for demodulating signals from the apparatus. It relies on synchronously digitizing the modulated signal at four times the modulation rate. It has the advantage that it can be easily adapted for time-domain demodulation using the host computer or digital signal processor (DSP) technology. Signals carried at harmonic components of the modulation frequency, normally lost in standard demodulation methods, are aliased to lower frequencies in the digital approach, with a consequent increase in signal to noise ratio over methods which isolate the harmonic carriers separately. An alternative simple binary modulation scheme is also possible. This will require the use of two detectors, but has the advantage of conceptual simplicity and also halves the drive voltage amplitude (typically xcx9c500 Volts).
Many of the advantages of the invention flow from the use of an electro-optical birefringent crystal to simultaneously provide a large electrically-tunable dc phase shift xcfx860 and a modulation component of variable frequency and adjustable amplitude xcfx860. The modulation can be sinusoidal (for single detector applications) or binary (requiring two detectors) with resulting post processing simplifications. These advantages include:
The present invention requires only a single wavelength-specific componentxe2x80x94the interference filter, compared with five in the Stenflo prior art device.
The number of independent optical components required is four rather than the ten of the Stenflo prior art device.
The modulation frequency in the present invention can be user selectedxe2x80x94it is not fixed by the resonant photo-elastic effect as in prior art devices.
The present invention avoids the problem of the Stenflo device in relation to beamsplitter introduced relative phase disturbances which limit the ultimate temperature resolution of that instrument.
Some further advantages of the present invention include:
1. A single detector, for example, a photomultiplier tube only is required to simultaneously recover the three lowest order spectral moments.
2. The instrument is low cost. In experimental applications, this will allow a number of spectral lines to be monitored routinelyxe2x80x94a prohibitively expensive option using dispersive spectrometers and detector arrays.
3. The apparatus of the present invention is robust (non-critical alignment) and compact. The modular system design allows the spectrometers to be combined for simultaneous multiple applications. For example, for plasma emission in coronal equilibrium and with the appropriate choice of interference filters and coupling optics, the instrument can be configured for sensitivity to electron temperature.
4. The modulation frequency can be selected to match some characteristic system frequency (e.g. to study wave phenomena) to allow the application of synchronous detection techniques.
5. Additional birefringent crystals can be inserted with axes either aligned with the modulating crystal (additive), or orthogonal (subtractive), to allow gross adjustment of the phase delay xcfx860. This is useful for applications exhibiting a wide range of temperatures.
6. The apparatus of the invention typically uses a 25 mm aperture which allows, with some modification of the detection scheme, the multiplexing of spatially distinct information larger apertures can be used in imaging applications.
7. The Zeeman effect (important for magnetized media) can be readily measured with the insertion of appropriate polarizing optics between the source and the spectrometer.
In the preferred form the instrument of this invention will be supplied with a PC card having computer controlled onboard oscillator, digitizer clock signal, filters and adc(s). The card will come with a graphical interface software package necessary for operation, control and demodulation. Some of the features of the combined system are:
Operation in oscillatory or binary modulation modes.
Possibility for continuous time monitoring for process control. Fixed data records can also be recorded and post processed by the PC.
User-controllable modulation frequency
Photomultiplier tube sensitivity control
As output, the PC card will provide
1. Low voltage supply for modulator.
2. Master oscillator signal (set by user)
3. Adjustable high voltage for photomultiplier.
The low voltage supply, together with the oscillator signal will be used to provide a switchable high voltage for modulation of the electro-optic crystal. An independent computer controllable supply could be provided for the photomultiplier tube detectors. If required, the high voltage could be slaved to ensure optimum use of the ADC dynamic range.
Inputs to the card will be the photodetector signals. These will be digitized synchronously as described above and processed by the computer.
Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.